In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film.
A resist material in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength (increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in mass production. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter (energy higher) than these excimer lasers, such as electron beam, extreme ultraviolet radiation (EUV), and X ray.
Resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources.
Conventionally, as a resist material that satisfies these conditions, a chemically amplified composition is used, which includes an acid-generator component that generates acid upon exposure and a base material component that exhibits a changed solubility in a developing solution under the action of acid.
As the base component used in a chemically amplified resist composition, a resin (base resin) is generally used.
For example, in the case of forming a positive-tone resist pattern by an alkali developing process using an alkali developing solution as the developing solution, a chemically amplified resist which contains an acid generator and a resin composition that exhibits increased solubility in an alkali developing solution by the action of acid is generally used. If the resist film formed using the resist composition is selectively exposed during formation of a resist pattern, then within the exposed portions, acid is generated from the acid-generator component, and the action of this acid causes an increase in the solubility of the resin component in an alkali developing solution, making the exposed portions soluble in the alkali developing solution. Thus, by conducting alkali developing, the unexposed portions remain to form a positive resist pattern.
As the resin component, a resin that exhibits increased polarity by the action of acid is generally used. When the polarity increases, the solubility of the resin in an alkali developing solution is increased, whereas the solubility of the resin in an organic solvent decreases. Therefore, when such a base resin is applied to a solvent developing process using a developing solution containing an organic solvent (organic developing solution) instead of an alkali developing process, the solubility of the exposed portions in an organic developing solution is decreased. As a result, in the solvent developing process, the unexposed portions of the resist film are dissolved and removed by the organic developing solution, and a negative resist pattern in which the exposed portions are remaining is formed. Such a solvent developing process for forming a negative-tone resist composition is sometimes referred to as “negative-tone developing process” (for example, see Patent Document 1).
Currently, resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are now widely used as base resins for resist compositions that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm (for example, see Patent Document 2).
Here, the term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position. The term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position.
The base resin contains a plurality of structural units for improving lithography properties and the like. For example, in the case of a positive-type resist, a base resin containing a structural unit having an acid decomposable group that is dissociated by the action of acid generated from the acid-generator component (e.g., a group that contains an acid dissociable group), a structural unit having a polar group such as a hydroxy group, a structural unit having a lactone-ring structure (—O—C(O)—), and the like is typically used. In recent years, instead of a structural unit containing a lactone-ring structure, a structural unit containing a sultone ring (—O—SO2—) has been used. These structural units enhance the adhesion to a substrate, and contribute to suppressing pattern collapse, thereby attracting attention (see for example, Patent Document 3).